1. Field of the Invention
The present invention relates to an image generating apparatus, and more particularly, it relates to an image generating apparatus comprising a print head for printing.
2. Description of the Background Art
An image generating apparatus actuating a pressing mechanism for pressing a print head against a platen roller with a gear train or a link mechanism is known in general, as disclosed in Japanese Utility Model Laying-Open No. 62-139658 (1987), Japanese Patent Laying-Open No. 9-188027 (1997) or Japanese Patent Laying-Open No. 9-188029 (1997), for example.
The aforementioned Japanese Utility Model Laying-Open No. 62-139658 discloses a thermal printer (image generating apparatus) having an arm, employed for rocking a thermal head (print head), mounted on a gear to interlock with rotation of the gear by coming into contact with a pin provided on the gear, for pressing the print head against a platen roller by rotating the gear with a stepping motor. This thermal printer applies a constant torque pressure to the print head by transmitting the same between the stepping motor and the gear mounted with the arm with a friction clutch. This friction clutch comprises a gear, a spring for producing frictional force and a spring clamp for clamping the spring.
The aforementioned Japanese Patent Laying-Open No. 9-188027 discloses a thermal transfer printer (image generating apparatus) actuating a link mechanism through rotation of a cam for applying a constant load resulting from a tension spring to a line thermal head and further rotating the cam thereby driving another mechanism. The link mechanism of this thermal transfer printer comprises a link portion formed by first and second links for pressing a print head against a platen roller, a crank driving the link portion, a pressure arm having the tension spring for applying pressing force to the link portion and the cam for driving the crank.
The aforementioned Japanese Patent Laying-Open No. 9-9-188029 discloses a pressure support engaging a locking pin of a locking lever with a notched hole provided on a first end of an L arm thereby coupling the L arm with a print head and applying a spring load of a tension spring mounted on a second end of the L arm as pressing force for pressing the print head against a platen roller. This pressure support additionally comprises a pressure switching cam mechanism since the same cannot be switched between a pressing position engaging the locking pin with the notched hole of the L arm and a pressing release position disengaging the locking pin from the notched hole of the L arm with only a pressing member formed by the L arm.
However, the conventional image generating apparatus described in the aforementioned Japanese Utility Model Laying-Open No. 62-139658, Japanese Patent Laying-Open No. 9-188027 or Japanese Patent Laying-Open No. 9-188029 actuating the pressing mechanism pressing the print head against the platen roller with the gear train or the link mechanism must be provided with the friction clutch (Japanese Utility Model Laying-Open No. 62-139658) constituted of large numbers of gear trains and components, the link mechanism (Japanese Patent Laying-Open No. 9-188027) having a complicated structure or the pressure switching cam mechanism (Japanese Patent Laying-Open No. 9-188029) independent of the L arm and the spring forming the pressing mechanism. Therefore, the number of components is increased, and the structure is complicated.
In this regard, there has been proposed an image generating apparatus whose structure is simplified by directly employing a cam for pressing a print head against a platen roller without a link mechanism or the like, as disclosed in Japanese Patent Laying-Open No. 5-286198 (1993) or Japanese Patent Laying-Open No. 6-15852 (1994), for example.
The aforementioned Japanese Patent Laying-Open No. 5-286198 discloses a thermal transfer printer (image generating apparatus) provided with a spring and a stopper between a print head pressing lever and a print head for integrally actuating the print head pressing lever and the print head and pressing the print head against a platen roller by rotating the print head pressing lever with the cam. When a cam pin of the print head pressing lever engaging with the cam reaches the outer periphery of the cam in this thermal transfer printer, the print head separates from the platen roller due to rotation of the print head pressing lever. When the cam pin of the print head pressing lever reaches the inner periphery of the cam, on the other hand, the print head comes into contact with the platen roller due to rotation of the print head pressing lever while the print head pressing lever and the stopper provided on a print head lever separate from each other, so that the print head is pressed against the platen roller due to urging force of a compression spring provided on the print head pressing lever. Thus, the print head rotates by the difference between the radial lengths of the outer and inner peripheries of a cam groove in the thermal transfer printer according to Japanese Patent Laying-Open No. 5-286198.
The aforementioned Japanese Patent Laying-Open No. 6-15852 discloses a printer capable of pressing and separating a print head against and from a platen roller with a cam and an L arm. When a cam pin of the L arm engaging with the cam reaches the outer periphery of the cam in this printer, a pin provided on the L arm and a print head lever come into contact with each other so that the L arm lifts up the print head lever. Thus, the print head is separated from the platen roller. When the cam pin of the L arm engaging with the cam reaches the inner periphery of the cam, on the other hand, the pin provided on the L arm and the print head lever separate from each other so that the spring load of a torsion spring provided between the L arm and the print head lever is applied to the print head. Thus, the print head is pressed against the platen roller. Also in the printer according to Japanese Patent Laying-Open No. 6-15852, the print head rotates by the difference between the radial lengths of the outer and inner peripheries of a cam groove.
In the thermal transfer printer (printer) disclosed in the aforementioned Japanese Patent Laying-Open No. 5-286198 or 6-15852, however, the print head is pressed against the platen roller with the general cam, and hence the rotational angle of the print head disadvantageously substantially corresponds to the radius of the print head. Consequently, the cam is disadvantageously hard to use when it is intended to actuate the print head at a large rotational angle exceeding the radius of the cam.